Methods of sealing anodic aluminium



METHODS OF SEALING ANODIC ALUMINIUM OXIDE COATINGS Roy C. Sp'ooner, Kingston, Ontario, Canada, assignor to Aluminium Laboratories Limited, Montreal, Quebec, Canada, a corporation of Canada No Drawing. Application July 29, 1957 Serial No. 674,562

Claims priority, application Great Britain July 30, 1956 Claims. (Cl. 204-35) The present invention relates to a method of sealing anodic aluminium oxide coatings formed on aluminium or aluminium-base alloys.

It is common practice to apply a sealing treatment to such anodic aluminium oxide coatings for the purpose of rendering the coating less porous and less permeable to penetration by substances which can corrode the underlying aluminium.

A very simple treatment which is satisfactory for most ordinary purposes is to immerse the coated metal in boiling water for a period of about 30 minutes. This treatment is not, however, sufiicient to seal the coating in an entirely satisfactory manner where the coated metal is later to be exposed to external atmospheric conditions. Where it is desired to render an anodic aluminium oxide coating particularly corrosion-resistant, it is already known to seal the coating by treatment in a solution containing chromates; for example, by immersion for 30 ice is due to the presence in the sealing bath of the molybdate ion M00 and this is said to be stable in solutions having a pH value of 6.5 and over, but it has been found in practice that satisfactory results are obtained with the pH value of the sealing bath held between 5.8 and 8.

Using sodium molybdate (N-a MoO :2H O), it is found that the preferred range of concentration of the salt in the sealing bath is 0.1-5 (preferably about 2%) held at a pH value between 5.8 and 8.0, preferably 6.5-7.0. The pH value of the bath may be adjusted to the desired value by adding molybdic acid or sodium hydroxide.

Similar results may be obtained using potassium molybdate (K MoO solutions, but sodium molybdate is preferred by reason of its lower cost. The commercially available ammonium paramolybdate may also be employed with satisfactory results. Also mixtures of two or more of these salts may be employed in the sealing bath. Again for commercial reasons a bath containing 2% sodium molybdate is preferred to a stronger bath, since it gives equally good results.

The coated metal to be sealed is preferably immersed in the bath for 5-30 minutes whilst the temperature of the bath is held approximately at boiling point; the necessary time for sealing will increase with the depth of the anodic coating.

The results of one comparative test carried out on aluminium panels all anodised under the same conditions and then sealed in sealing baths containing water alone, a 2% sodium molybdate solution and a 5% potassium dichromate solution, are given below.

Water sealing Sodium molybdate (2O g./1.) Potassiurggiclilrgimate Sealing time to an 30, Sealing em 98- 98-100 98-100. Sealant pH 6.1. 6.7.. Colour sealed panel Light grey Light grey Pronounced olive green. Weight loss during 6 hrs. immersion in 7Q 11 38.

cold sulphuric acid (mgJdmJ). Cathodic current value (ma/(1m 2 2 Intermittent acidified salt spray 110 days exposure.

1.4 2.1. Very slight pitting attack. Very slight pitting attack.

minutes in a 5% solution of potassium dichromate at 98-100 C.

This treatment greatly increases the corrosion and weathering resistances of the anodically coated metal, but unfortunately the treatment imparts an olive green shade to the anodic coating, which makes the metal thus treated less acceptable for purposes, such as curtain walling, than it would be if it retained the ordinary grey colour of Water-sealed anodised aluminium.

It is an object of the present invention to provide a new method of sealing an anodic aluminium oxide coating to render it appreciably more resistant to corrosion and weathering than could be achieved by a water sealing treatment, Whilst at the same time avoiding the undesirable colouring of the anodic coating which arises when the coating is sealed in a chromate solution.

According to the present invention a method of sealing anodically formed aluminium oxide coatings comprises immersing the metal on which the coating is formed in a bath containing a solution of an alkali metal or ammonium salt of molybdic acid held at a temperature above 90 C.

The sealing treatment of anodic coatings with an alkali metal molybdate leaves the coatings with a final light grey colour similar to that achieved when an anodic coating is sealed in boiling water.

It is believed that the improved effectiveness of sealing I claim:

1. A method of sealing anodically formed aluminium oxide coatings on a metal selected from the group consisting of aluminium and aluminium-base alloys comprising immersing the coated metal in a bath containing an aqueous solution of at least one of the group consisting of sodium molybdate, potassium molybdate and ammonium paramolybdate, the said solution being held at a pH value within the range 5.8-8.0 and a temperature above C.

2. The method of claim 1, wherein the pH value is held within the range '6.5-7.0.

3. The method of claim 1, wherein the bath solution contains 0.1-5% sodium molybdate.

4. The method of claim 3, wherein the bath solution contains about 2% sodium molybdate.

5. The method of claim 1, wherein the bath solution is held substantially at boiling point and the coated metal is immersed therein for 5-30 minutes.

References Cited in the file of this patent UNITED STATES PATENTS 2,313,755 Loose Mar. 16, 1943 2,448,513 Brennan Sept. 7, 1948 2,574,225 Ringk et a1. Nov. 6, 1951 2,723,952 Evangelides Nov. 15, 1955 

1. A METHOD OF SEALING ANODICALLY FORMED ALUMINIUM OXIDE COATINGS ON A METAL SELECTED FROM THE GROUP CONSISTING OF ALUMINIUM AND ALUMINIUM-BASE ALLOYS COMPRISING IMMERSING THE COATED METAL IN A BATH CONTAINING AN AQUEOUS SOLUTION OF AT LEAST ONE OF THE GROUP CONSISTING OF SODIUM MOLYBDATE, POTASSIUM MOLYBDATE AND AMMONIUM PARAMOLYBDATE, THE SAID SOLUTION BEING HELD AT A PH VALUE WITHIN THE RANGE 5.8-8.0 AND A TEMPERATURE ABOVE 90*C. 